386 



temperature, however, a definite liquid Lj cannot be in equilibrium 

 with vapour under two different pressures. 



P and P' must therefore be the same. Further it also follows that 

 the vapours G and G' are the same. We call this vapour G/^ . 



The point of intersection of the two saturation curves under their 

 own vapour pressures signifies, therefore, that under a 'definite pres- 

 sure the three-phase equilibrium F-\- Lf-\- G/^ can be met with as well 

 as F' -\- Lf-\- Gf^. To every point of intersection ƒ of the saturation 

 curve belongs, therefore, a detinite point of intersection f\ of the 

 vapoursatu ration curves under their own vapour pressures. Hence it 

 follows that the point of intersection ƒ represents the liquid L, the 

 corresponding point of intersection f\ the vapour G of the four- 

 phase equilibrium F -\- F' -\- L -\- G ; in other words : ƒ represents 

 the solution saturated with F -\- F' under its own vapour pressure; 

 /i is the corresponding vapour. The same, of course, applies to the 

 points of intersection g and g^. 



The following equilibria, therefore, exist in fig. 2 : 



1. A series of solutions saturated with F under their own Vpipour 

 pressures, wdth their corresponding vapours; therefore the system 

 FJ^L-^G. 



2. A series of solutions saturated with F' under their own vapour 

 pi'essures, with their corresponding vapours ; therefore the system 

 F' ^L-Y G. 



3. Two solutions saturated with P^ -\- F' under their own vapour 

 pressures with their corresponding vapours ; therefore the system 

 F -\- F' -{- L -\- G. The one exists under the pressure Pj with the 

 liquid ƒ and the vapour f^ ; the other under the pressure Py with 

 the liquid g and the vapour g^. Usually P/- and P,, are different; 

 only in exceptional cases can they be equal. 



For the sake of abbreviation we shall call in future the point of 

 intersection of two saturation curves under their own vapour pres- 

 sures "the liquid point of intersection", and the point of intersection 

 of two vapour saturation curves under their own vapour pressures 

 "vapour point of intersection". 



If we now assume, that in the figs. 1 — 3 (VIII) the saturation 

 and vapour saturation curves of 7" and of F' under their own 

 vapour pressures are drawn, then it is evident that the liquid points 

 of intersection fall on the liquid cur\e ac and the vapour points of 

 intersection on the vapour curve cïiCi of the four-phase equilibrium. In 

 fig. 'J, corresponding with fig. 1 (VIII) some of these curves are 

 partly drawn. On the liquid curve ac [fig. 1 — 3 (VI II J we take a 

 point ^V, corresponding with the point of maximum temperature N 



